It is generally found that a protein performing a certain bioactivity exhibits a certain variation between genera and even between members of the same species differences may exist. This variation is of course even more outspoken at the genomic level.
This natural genetic diversity among genes coding for proteins having basically the same bioactivity has been generated in Nature over billions of years and reflects a natural optimization of the proteins coded for in respect of the environment of the organism in question.
In today's society the conditions of life are vastly removed from the natural environment and it has been found that the naturally occurring bioactive molecules are not optimized for the various uses to which they are put by mankind, especially when they are used for industrial purposes.
It has therefore been of interest to industry to identify such bioactive proteins that exhibit optimal properties in respect of the use to which it is intended.
This has for many years been done by screening of natural sources, or by use of mutagenesis. For instance, within the technical field of enzymes for use in e.g. detergents, the washing and/or dishwashing performance of e.g. naturally occurring proteases, lipases, amylases and cellulases have been improved significantly, by in vitro modifications of the enzymes.
In most cases these improvements have been obtained by site-directed mutagenesis resulting in substitution, deletion or insertion of specific amino acid residues which have been chosen either on the basis of their type or on the basis of their location in the secondary or tertiary structure of the mature enzyme (see for instance U.S. Pat. No. 4,518,584).
In this manner the preparation of novel polypeptide variants and mutants, such as novel modified enzymes with altered characteristics, e.g. specific activity, substrate specificity, thermal, pH and salt stability, pH-optimum, pI, K.sub.m, V.sub.max etc., has successfully been performed to obtain polypeptides with improved properties.
For instance, within the technical field of enzymes the washing and/or dishwashing performance of e.g. proteases, lipases, amylases and cellulases have been improved significantly.
An alternative general approach for modifying proteins and enzymes has been based on random mutagenesis, for instance, as disclosed in U.S. Pat. No. 4,894,331 and WO 93/01285
As it is a cumbersome and time consuming process to obtain polypeptide variants or mutants with improved functional properties a few alternative methods for rapid preparation of modified polypeptides have been suggested.
Weber et al., (1983), Nucleic Acids Research, vol. 11, 5661, describes a method for modifying genes by in vivo recombination between two homologous genes. A linear DNA sequence comprising a plasmid vector flanked by a DNA sequence encoding alpha-1 human interferon in the 5'-end and a DNA sequence encoding alpha-2 human interferon in the 3'-end is constructed and transfected into a rec A positive strain of E. coli. Recombinants were identified and isolated using a resistance marker.
Pompon et al., (1989), Gene 83, p. 15-24, describes a method for shuffling gene domains of mammalian cytochrome P-450 by in vivo recombination of partially homologous sequences in Saccharomyces cerevisiae by transforming Saccharomyces cerevisiae with a linearized plasmid with filled-in ends, and a DNA fragment being partially homologous to the ends of said plasmid.
In WO 97/07205 a method is described whereby polypeptide variants are prepared by shuffling different nucleotide sequences of homologous DNA sequences by in vivo recombination using plasmidic DNA as template.
U.S. Pat. No. 5,093,257 (Assignee: Genencor Int. Inc.) discloses a method for producing hybrid polypeptides by in vivo recombination. Hybrid DNA sequences are produced by forming a circular vector comprising a replication sequence, a first DNA sequence encoding the amino-terminal portion of the hybrid polypeptide, a second DNA sequence encoding the carboxy-terminal portion of said hybrid polypeptide. The circular vector is transformed into a rec positive microorganism in which the circular vector is amplified. This results in recombination of said circular vector mediated by the naturally occurring recombination mechanism of the rec positive microorganism, which include prokaryotes such as Bacillus and E. coli, and eukaryotes such as Saccharomyces cerevisiae.
One method for the shuffling of homologous DNA sequences has been described by Stemmer (Stemmer, (1994), Proc. Natl. Acad. Sci. USA, Vol. 91, 10747-10751; Stemmer, (1994), Nature, vol. 370, 389-391). The method concerns shuffling homologous DNA sequences by using in vitro PCR techniques. Positive recombinant genes containing shuffled DNA sequences are selected from a DNA library based on the improved function of the expressed proteins.
The above method is also described in WO 95/22625. WO 95/22625 relates to a method for shuffling of homologous DNA sequences. An important step in the method described in WO 95/22625 is to cleave the homologous template double-stranded polynucleotide into random fragments of a desired size followed by homologously reassembling of the fragments into full-length genes.
A disadvantage inherent to the method of WO 95/22625 is, however, that the diversity generated through that method is limited due to the use of homologous gene sequences (as defined in WO 95/22625).
Another disadvantage in the method of WO 95/22625 lies in the production of the random fragments by the cleavage of the template double-stranded polynucleotide.
A further reference of interest is WO 95/17413 describing a method of gene or DNA shuffling by recombination of specific DNA sequences--so-called design elements (DE)--either by recombination of synthesized double-stranded fragments or recombination of PCR generated sequences to produce so-called functional elements (FE) comprising at least two of the design elements. According to the method described in WO 95/17413 the recombination has to be performed among design elements that have DNA sequences with sufficient sequence homology to enable hybridization of the different sequences to be recombined.
WO 95/17413 therefore also entails the disadvantage that the diversity generated is relatively limited. Furthermore the method described is time consuming, expensive, and not suited for automatisation.
Despite the existence of the above methods there is still a need for better iterative in vitro recombination methods for preparing novel polypeptide variants. Such methods should also be capable of being performed in small volumes, and amenable to automatisation.